(A) Assessment of histone H2AX phosphorylation (γH2AX) at 1 and 24 hours postirradiation (2 Gy) by MFI ratio. The mode ratio of the MFI of γH2AX (irradiated [IR] to unirradiated [non-IR]) in T-cell and NK cell subsets is shown. (B) Frequencies of T and NK cells expressing histone H2AX phosphorylation (γH2AX) 1 and 24 hours postirradiation (2 Gy). The frequency of NK cells with dephosphorylated H2AX at 24 hours remains essentially unchanged (but there is a reduced MFI ratio). (C) Visual depiction of flow cytometric analysis showing γH2AX at 1 and 2 hours postirradiation in a representative control and patient. Patient T cells show 2 peaks for CD3⁺ T cells at 1 hour postirradiation (absent at 24 hours). Approximately two-thirds of T cells still have γH2AX at 24 hours (reduced MFI). About a third of T cells show complete dephosphorylation at 24 hours. The majority of NK cells still have γH2AX, although there is evidence of partial dephosphorylation.